(1) Field of the Invention
The present invention relates to a visually effective image switching apparatus for producing an image switching special effect by synthesizing digital image data corresponding to two images. The images are synthesized when an image displayed on an image display device is switched with another image to be displayed.
(2) Description of the Prior Art
There are several examples of prior art devices, for example, "Special Effect Generating device, Japanese Patent Laid-Open 63-197178", "Video Signal Processor, Japanese Patent Laid-Open 1-190176", and "Video Special Effect Processing System, Japanese Patent Laid-Open 3-267885", each of which is incorporated herein by reference, which relate to special effect processing. Another example of a related prior art device is discussed in U.S. Pat. No. 4,197,590 entitled "Method for Dynamically Viewing Image Segments Stored in a Random Access Memory Array", which is incorporated herein by reference.
A conventional special effect generating apparatus for synthesizing two pictures has a temporary memory for storing at least two pictures of digital image data to be synthesized.
Each digital image data is read from the temporary memory. Then (1) the data is calculated for synthesis and D/A converted to an analog video signal of one frame, or (2) each image is D/A converted and then each image data is calculated for synthesis. The series of processing steps which generate a video signal of one frame is regarded as a unit. A unit is continually repeated to produce the special effect of synthesizing two images. A unit is the series of steps necessary to generate one frame of a video signal. When a unit is repeated successively, momentary variation can be added to the special effect which is obtained by switching the display picture. This is accomplished by changing or modifying the parameters and the method used to calculate the synthesis every time the steps are repeated to create a unit.
When the minimum time required to perform the processing steps for one unit is reduced, a smoother and more effective special effect by synthesizing two images is obtained.
For example, one frame of a television video signal is displayed every 1/30 second. As a result, a calculation device for synthesizing images must operate at a high speed. In addition, a temporary memory composed of semiconductor memory devices is also needed so that data can be supplied at a high rate of speed.
The digital image data is usually stored in an external storage device having a large capacity such as a filing unit. However, the amount of data in an image is generally large and the data reading speed of external storage devices having a large capacity is very slow as compared with that of a semiconductor memory device. As a result, a video signal of one frame cannot be generated at high speed, when using an external storage device without a buffering memory such as the temporary memory mentioned above. As a consequence, a smooth and effective special effect cannot be realized.
The temporary memory which is used as a buffer memory to store digital image data is, however, generally assembled as a frame memory unit. A frame memory unit is composed of expensive and large scale semiconductor integrated circuits. As a result, a special effect generating apparatus using the frame memory unit is expensive and large.
The image data for one picture is typically read according to a refresh frequency of the display device.
Moreover, if a dual port RAM which can read and write image data simultaneously is used, the expense and size of the special effect generating apparatus is increased. In contrast, if DRAMs are used, the expense and size of the apparatus is reduced.